The electrochromic glazings according to the invention are glazings having a layer of a material able to reversibly insert cations, generally lithium ions or protons and whose oxidation states corresponding to the two inserted or deinserted states have different colouring states. In the case of tungsten trioxide, there is consequently a passage from a colourless oxide state into a reduced midnight blue colouring state, in accordance with the chemical reaction: EQU WO.sub.3 +xM.sup.+ +xe.sup.- M.sub.x WO.sub.3.
In order that the said reaction takes place, it is necessary to have alongside the electrochromic material layer a source of cations and a source of electrons, respectively constituted by a layer of an electrolyte having an ionic conductivity and an electroconducting layer serving as the electrode. To this first system incorporating a cathodic electrochromic material is added, by symmetry, a system with an anodic electrochromic material, such as iridium oxide, able to insert and deinsert cations in a reversible manner, the iridium oxide layer being inserted between the electrolyte layer and a second transparent electroconductive layer.
In European patent applications EP-A-253 713 and EP-A-338 876, the relationship existing between the size of an electrochromic system and the electric conductivity required for the electrode is shown. The switching times are increased unless the glazing has large dimensions and the transparent electroconductive layer has a low resistivity. For glazings larger than 200 cm, a square resistance below 5 Ohms is consequently recommended.
Such performance characteristics can certainly not be obtained with any type of thin layer, particularly as the other fundamental requirement is the transparent nature, a maximum light transmission being sought for the colour-removed state, which involves relatively small thicknesses in the case of numerous materials.
To this double requirement of high performance characteristics for the electrical conductivity and transparency is added in the case of glazings typically intended for use in cars and in particular sunroofs, the compatibility with a heat treatment of the glass sheet of the cambering and/or annealing type. This compatibility can be obtained with a camberable layer, i.e. able to withstand without deterioration a treatment at 600.degree. C., or with a layer which can be deposited on a cold substrate, the reheating of the glass may lead to it losing its shape and the beneficial effects of the annealing process. Finally, the technology used must be adapted to the dimensions of the glazings and to industrial production, particularly from the cost and efficiency standpoints.
In the state of the art relative to electroconductive material layers, it would appear that the only suitable layers are those based on doped metallic oxides and more particularly layers of tin-doped indium oxide (ITO). Thus, the electrochromic systems proposed are typically stacks of glass/ITO/WO.sub.3 (or some other cathodic electrochromic material)/electrolyte/Ir.sub.2 O.sub.3 (or some other anodic electrochromic material)/ITO/glass.
However, in such systems, it has been found that there are problems linked with interactions between the electroconductive layers and the electrolyte or more precisely the cations which it supplies. Such a phenomenon can appear surprising to the extent that the transparent electroconductive layer is not directly in contact with the electrochromic material. Nevertheless, this intercalated layer can never completely fulfil a protective function, because it must offer a maximum number of reactive sites for the cations, i.e. with a distribution over the entire thickness of the electrochromic material layer, which implies a relatively porous layer, which will be penetrated by the cations and which can therefore reach the transparent electroconductive layer.
In the case of proton systems based on the reversible insertion of protons, the deterioration of the system would appear to be linked with the slow corrosion mechanism due to acid-base reactions, even if the electrolyte is chosen from among the non-corrosive types, such as e.g. a solid acid-organic polymer complex. However, a limitation will be made hereinafter to electrochromic systems not using liquid acid solutions, whose corrosive nature is prejudicial to all the layers of the system and in particular to the electrochromic material layers. It should be noted that proposals have been made to substitute these polymer materials by materials such as dielectrics, but the latter have to be deposited in extremely thin film form, because if not their own colouring becomes prejudicial and in particular leads to risks of short-circuits in the case of large systems. In any case, cations must be present in the system and these substitution materials lead to a reduction of their numbers.
In the case of systems based on the reversible insertion of lithium ions, this problem of acid/base reactions a priori does not exist, so that a longer life of the systems could be expected. However, an ageing of the electrodes formed from doped indium oxide has been observed, said electrodes gradually turning brown. Research carried out by various authors has revealed that this colouring is probably due to a cathodic electrochromism phenomenon of the indium oxide, but unlike in the case of "true" electrochromic materials, this phenomenon is not totally reversible, so that the colouring persists.
It should be noted that this corrosion phenomenon is relatively slow and that the system normally functions for a certain time. However, following a long period of use or a simulation thereof by accelerated ageing tests at high temperatures, there is a significant deterioration e.g. after 5 hours at 100.degree. C. or a higher temperature. Therefore this corrosion is a handicap to the development of systems, particularly in the case of building applications where there is a ten-year guarantee problem and in car applications where the use temperatures are often very high.
The invention aims at a novel type of electrochromic glazing, which does not suffer from this deterioration phenomenon with respect to the electroconductive layers.